Mammalian cell culture that other people can reproduce: authentication and aseptic work

The practices that decide whether the work is reproducible

1. Authenticate the line, then authenticate it again. Short tandem repeat profiling identifies a human line against a reference database and is inexpensive. Do it when the line arrives, before any publication and after any incident, because misidentified and cross-contaminated lines remain a substantial fraction of what circulates between laboratories.
2. Test for mycoplasma on a schedule, not on suspicion. Mycoplasma does not make the medium cloudy and changes gene expression, growth and drug response profoundly. Test at least quarterly and on every new arrival, quarantine incoming lines until they pass, and treat a positive as a reason to discard rather than to treat.
3. Bank in two tiers and work from the second. A master bank frozen at low passage and a working bank expanded from it. Everything for the study comes from the working bank, and when it is exhausted a new one is made from the master. Working continuously from a thawed vial guarantees drift nobody can measure.
4. Decide serum against a defined medium deliberately. Serum is undefined, varies between lots and carries supply risk; it also supports a great many lines that a defined medium does not. If you use it, test and reserve a lot. If you can move to a defined or serum-free formulation, the reproducibility gain is real and the adaptation takes weeks.
5. Record passage number with every experiment. Cells change with passage, and results from passage eight and passage forty are not comparable. Recording it costs nothing and is the single most useful number in a cell culture record.

Aseptic technique and the incubator

Most contamination arrives on hands and on bottles, not through the air. Work in a cabinet that has been certified, decontaminate everything entering it, and do not share a bottle of medium between people or between lines.

Clean and check the incubator on a schedule, including the water tray, and do not run antibiotics continuously. Routine antibiotics mask poor technique and select for resistant contamination that appears later and worse.

Freezing and thawing without losing the line

Freeze at high viability in the medium the line grows in with a cryoprotectant, cool slowly, and move to vapour phase nitrogen for long term storage. Thaw quickly and dilute out the cryoprotectant promptly.

Record the vial position, the passage and the viability at freezing. A bank nobody can navigate is a bank that gets thawed from the top, which is not the same as being used systematically.

What to write in a methods section

The line's source and catalogue identifier, the authentication method and date, the mycoplasma status, the medium and supplements with the serum lot where relevant, and the passage range used. That paragraph is what lets somebody else repeat the work.

Where a line was modified, state how and give the clone identity. A named parental line and an unnamed derivative is the most common gap in an otherwise complete methods section.

From a flask to cell culture process development

Growing cells to keep them and growing cells to make something are different activities with different success criteria. The first is judged on health and consistency; the second on product quantity, product quality and reproducibility from a defined bank, and almost everything about the medium and the vessel changes with that shift.

Development work is normally staged: establish the host and the construct, define a medium and feed, then scale in a vessel where the parameters can be controlled and recorded. Skipping the middle stage is the usual reason a scale-up produces less than the flask predicted.

Protein expression in mammalian cells: why the host is chosen at all

Bacterial expression is faster and cheaper and cannot add most of the modifications a secreted human protein carries. Where those modifications affect folding, activity, stability or immunogenicity, the host has to be a mammalian cell and the cost difference is the price of the modification.

So the first question is not which host yields most but which modifications the product needs. A protein that does not need them should not be made in a mammalian system, and one that does cannot be made anywhere else.

Fed batch, perfusion and the vessel that follows

A fed batch adds nutrients to a fixed volume and harvests at the end, which is simple and limits duration. Perfusion continuously exchanges medium while retaining cells, which sustains higher densities for longer and suits an unstable product that should not sit in the vessel.

Perfusion costs more medium, needs a retention device and is harder to control, so it is chosen when the product demands it rather than for throughput alone. That decision determines the vessel, the medium consumption and the level of instrumentation the laboratory has to own.

What a mammalian cell culture bioreactor actually requires

A stirred vessel with control of temperature, pH, dissolved oxygen and feeding is a different commitment from an incubator: probes that need calibration, gases, a controller, cleaning or single-use consumables, and records of every parameter for every run.

Single-use vessels remove the cleaning and validation burden and add a consumable cost per run. Which is cheaper depends on run frequency, and the answer changes as a programme moves from occasional development runs to routine production.

A bioreactor for mammalian cell culture, and what it must do gently

Mammalian cells shear, so every part of the vessel is chosen for gentleness: a marine or pitched impeller at a low tip speed, sparging through a fine sparger or the headspace, carbon dioxide in the gas mix so pH is controlled without adding acid, and a temperature and osmolality window with little margin. Single use bags with rocking or stirred motion are common because cleaning validation is avoided. The number to compare between vessels is oxygen transfer at the shear the cells tolerate, not the volume.

A transfection kit, and choosing one for the cells you have

A transfection kit is chosen for the cell rather than for the cargo: an easy adherent line takes almost any cationic lipid or polymer, a primary or suspension culture usually needs electroporation or a reagent formulated for it, and a hard line sometimes needs a viral vector instead. The kit's protocol assumes a DNA stock at a stated concentration in a low salt buffer, free of endotoxin and of the RNA a rushed prep leaves, since the complex forms on whatever nucleic acid is present. Run a reporter beside the construct so the delivery is measured rather than assumed.

gentamicin cell culture use, and why routine use is discouraged

gentamicin cell culture use is common and is still not recommended as a routine additive, because a constant antibiotic masks a low grade contamination rather than preventing one and selects for resistance. Where it is used, it belongs in a rescue or a primary isolation, and the stock culture is kept antibiotic free so a failure of technique shows.

Common questions

How often should mammalian cell culture be tested for mycoplasma?

At least quarterly for lines in continuous use, and on arrival for every new line before it enters the main incubator. Infection is invisible without a test and changes results in ways that are easy to publish and hard to retract.

Do I really need to authenticate a line I got from a colleague?

Especially then. Lines passed between laboratories are the ones most likely to be misidentified, and a short tandem repeat profile is cheap compared with a study built on the wrong cells.

Serum or serum-free medium?

Serum-free where the line supports it, for reproducibility, defined composition and supply security. Serum where the line needs it, in which case test candidate lots against your own assay and reserve the one that works.

How many passages is too many?

It depends on the line, and the way to know is to measure: growth rate, morphology and the assay you care about, at intervals. Set a maximum passage from that evidence and return to the working bank when it is reached.

When is a mammalian host necessary?

When the product needs modifications a bacterial system cannot add, and those modifications affect folding, activity, stability or immunogenicity. If they do not, a simpler host is faster and cheaper.

Fed batch or perfusion cell culture?

Fed batch unless the product is unstable in the vessel or the density has to be sustained. Perfusion costs more medium, needs a cell retention device and is harder to control, so it is chosen for the product rather than for throughput.

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